1. Field of the Invention
The present invention relates to a device for detecting a state of thermal degradation of an exhaust purifying catalyst, detecting a state of degradation caused by heat, of an exhaust purifying catalyst arranged in an exhaust passage of an internal combustion engine for purifying exhaust gas.
2. Description of the Background Art
An exhaust purifying catalyst such as an NOx catalyst used for an internal combustion engine gradually degrades by heat through use, and its exhaust purifying function lowers. Therefore, it is important to exactly ascertain the state of thermal degradation of the exhaust purifying catalyst and to address any problem promptly.
For this purpose, various techniques for detecting any defect caused by thermal degradation of exhaust purifying catalyst have been conventionally proposed. In one such approach, rate of progress of thermal degradation (amount of thermal degradation per unit time) of the exhaust purifying catalyst is found, which is integrated for a predetermined time period to calculate the degree of thermal degradation, and presence/absence of any defect caused by thermal degradation is detected based on the degree of thermal degradation. In the technique disclosed in Japanese Patent Laying-Open No. 7-119447, p. 4 and FIGS. 3 and 4, noting the fact that the rate of progress of thermal degradation differs dependent on the temperature of exhaust purifying catalyst, the temperature of exhaust purifying catalyst is detected in every predetermined time period, and a degradation coefficient (rate of progress of thermal degradation) corresponding to the detected temperature is found from a map. In the map, when the temperature of exhaust purifying catalyst is in a relatively low temperature range, the degradation coefficient is set at a constant value regardless of the temperature. In a temperature range higher than that mentioned above, the degradation coefficient is set to increase in proportion to the increase of temperature of the exhaust purifying catalyst. The degradation coefficient found in this manner is integrated for every predetermined time period, and when the integrated value (degree of thermal degradation) exceeds a predetermined value, it is determined that the exhaust purifying catalyst is thermally degraded and defective.
The rate of progress of thermal degradation of exhaust purifying catalyst tends to be large when thermal degradation is not much developed (degree of thermal degradation is small) and to become smaller as thermal degradation proceeds (degree of thermal degradation increases), when the temperature of exhaust purifying catalyst is constant. Such a tendency might come from the following phenomenon. In the exhaust purifying catalyst, a large number of catalyst grains of noble metal are provided on a carrier, and the exhaust purifying performance varies in accordance with surface area of the catalyst. The exhaust purifying performance lowers as the surface area becomes smaller.
When the exhaust purifying catalyst is continuously exposed to high heat, catalyst grains come to adhere with each other. Such adhesion leads to reduced surface area. In other words, thermal degradation of exhaust purifying catalyst proceeds, and catalyst purifying performance degrades gradually.
When the thermal degradation is not much developed (degree of thermal degradation is small), grain size of each catalyst grain is small, and therefore, there is much margin for adhesion of catalyst grains to each other. Therefore, when exposed to high heat, large number of catalyst grains come to adhere to each other, so that the surface area decreases significantly. Specifically, thermal degradation of exhaust purifying catalyst proceeds at a high rate of progress. The margin for adhesion of catalyst grains to each other becomes smaller as thermal degradation proceeds (degree of thermal degradation increases). When thermal degradation has developed to some extent, the degree of surface area reduction becomes smaller than when thermal degradation is not much developed. In other words, thermal degradation of exhaust purifying catalyst proceeds at a lower rate of progress than when thermal degradation is not much developed. When development of thermal degradation reaches a certain point, change (reduction) in surface area practically stops.
As described above, the rate of progress of thermal degradation of the exhaust purifying catalyst has such a characteristic that it becomes smaller as the degree of thermal degradation becomes higher. Such a tendency is observed even when the temperature of exhaust purifying catalyst varies.
In Japanese Patent Laying-Open No. 7-119447, such a characteristic is not considered, and assuming that the rate of progress of thermal degradation is constant as long as the temperature of exhaust purifying catalyst is constant regardless of the degree of thermal degradation, the degradation coefficient corresponding to the temperature of exhaust purifying catalyst at that time (rate of progress of thermal degradation) is integrated for every predetermined time period. Therefore, there is still room for improvement for exactly ascertaining the degree of thermal degradation of the exhaust purifying catalyst.